ASTRONOMY 107 



SUBJECT MATTER OF ASTRONOMY 



Before going farther, in order to make more intelligible 

 what I shall say later on, I shall spend a few minutes in enum- 

 erating the classes of matter, the study of which constitutes 

 astronomy. We have first the solar system, which as you all 

 know consists of a central body, the sun, about which revolve 

 eight large bodies called planets, of which the earth is one of 

 the smallest. Some of these planets are themselves accom- 

 panied by one or more bodies called satellites, like our moon, 

 which revolve around the planets as they themselves revolve 

 around the sun. There are besides a large number of smaller 

 bodies called asteroids, which are really very small planets, 

 and occasionally a comet, an object which may become per- 

 manently attached to the solar system, like Halley's comet ; 

 but more often a comet merely enters the sysem for a short 

 time and then leaves, never to return. The whole solar system 

 covers a very large space, whose greatest dimension is about 

 6,000,000,000 miles ; but before reaching any of the other visible 

 heavenly bodies, one would have to travel about 30,000 times 

 as far as this, and then the distances from star to star are often 

 still many times greater. Some stars are so far from us that 

 it takes thirty or more years for their light to reach us. Since 

 light can travel seven times around the earth in a second, we 

 see that this means a considerable distance. 



These bright stars or suns differ greatly among them- 

 selves. Many of them are great composite systems like the 

 solar system but inconceivably bigger, some are merely pairs 

 of large bright bodies revolving about one another, while some 

 are probably single glowing spheres. Interspersed among 

 these are many dark bodies or extinct suns, which were once 

 brightly glowing, but which have cooled till they give no light 

 of their own. Of course we cannot see them, but we have very 

 good indirect evidence of the existence of many such bodies, 

 and we surmise that there must be a great many of them. 



